AVS 58th Annual International Symposium and Exhibition
    Surface Science Division Tuesday Sessions
       Session SS+EM-TuA

Paper SS+EM-TuA9
Direct Observation of NO2 Adsorption onto CuPc Monolayers with STM

Tuesday, November 1, 2011, 4:40 pm, Room 110

Session: Organic Electronic Interfaces
Presenter: JunHong Park, University of California San Diego
Authors: J.H. Park, University of California San Diego
J. Royer, University of California San Diego
S. Lee, University of California San Diego
T. Kent, University of California San Diego
W. Trogler, University of California San Diego
A.C. Kummel, University of California San Diego
Correspondent: Click to Email
Copper phthalocyanine (CuPc) thin film devices have been widely studied for use as chemical vapor sensors; however, the molecular scale sensing mechanism remains undetermined. This study presents molecular scale observation of NO2 adsorption onto CuPc monolayers using ultra-high vacuum (UHV) scanning tunneling microscopy (STM). CuPc monolayers were deposited on Au (111) surfaces by organic molecular beam epitaxy in ultra-high vacuum (UHV) and subsequently exposed to different NO2 concentrations at atmospheric pressure. After annealing at 50 ˚C to improve STM imaging, for low NO2 doses (1 ppm for 5min) the STM images reveal NO2 molecules arranged as islands primarily along the domain boundaries. The NO2 molecules almost completely desorb from the CuPc monolayer after annealing at 100 ˚C for 1 hr. Conversely, at high NO2 doses (10 ppm for 5 min), the NO2 exhibit irreversible reactions with the CuPc surface. After annealing at 50 ˚C, the domain boundaries act as nucleation centers for semi-ordered NO2 adsorbates. As the annealing temperature is increased to 150 ˚C, the islands of NO2 molecules diffuse from the domain boundaries to the CuPc terraces to form ordered structures on the terraces. After annealing at 250 ˚C for 1 hr, the CuPc monolayer becomes disordered and has numerous vacancies. The data is consistent with that NO2 inducing CuPc decomposition. The behavior of NO2 dosed CuPc monolayers suggests that NO2 undergoes molecular chemisorption with the CuPc metal center at low exposures. However, at high exposures, NO2 not only undergoes molecular chemisorption with the metal center, but also induces decomposition of the CuPc. This direct observation for NO2 dosed CuPc monolayers with STM can give an insight into reversible versus dosimetric sensing in CuPc organic thin film chemical sensors (chemFETs). While nearly all weakly bonding analytes give reversible mobility sensor responses on CuPc chemFETs, some strong oxidants are observed to give reversible mobility responses at short exposures and dosimetric irreversible threshold voltage responses for longer exposures. This is consistent with the larger doses saturating the reversible chemisorption sites and inducing a oxidative decomposition of the CuPc which induces a dosimetric response via formation of uncompensated positive charge in the CuPc film. Therefore, the data is consistent with two chemisorption mechanisms of a strong oxidant on a single molecule given two different classes of sensor response.